1. Field of the Invention
The present invention relates to a refrigeration lubricant composition. More specifically, the present invention relates to a refrigeration lubricant composition that is used for a chlorine-free hydrofluorocarbon refrigerant, wherein the composition comprises a polyol ester as the main component and has excellent low-temperature stability, lubricity, stability under coexistence with a chlorine-free hydrofluorocarbon refrigerant, and has excellent compatibility with chlorine-free hydroflurocarbon refrigerants. The present invention also relates to a refrigerant working fluid containing the refrigeration lubricant composition and a refrigerating apparatus including the refrigerant working fluid.
2. Description of the Related Art
Conventionally, refrigerants containing chlorofluorocarbon have been used for air conditioning equipment such as room air conditioners and packaged air conditioners, low temperature apparatuses such as refrigerator-freezers for home use, industrial refrigerators, and automotive air conditioners such as hybrid cars and electric cars. However, in recent years, the replacement of such chlorofluorocarbon refrigerants with chlorine-free hydrofluorocarbon refrigerants such as those containing 1,1,1,2-tetrafluoroethane (R-134a), pentafluoroethane (R-125), difluoromethane (R-32), and mixtures thereof has been promoted because of problems such as the depletion of the ozone layer. Consequently, a variety of refrigeration oils containing a polyol ester, which has good compatibility with chlorine-free hydrofluorocarbon refrigerants, as a base stock have been proposed.
In addition to the above-described compatibility with chlorine-free hydrofluorocarbon refrigerants, a variety of properties such as lubricity, thermal stability, hydrolytic stability, and low-temperature fluidity are required for refrigerating machine oils in order to ensure the stability of the above-described equipment. Among these, in view of hydrolytic stability and compatibility with chlorine-free hydrofluorocarbon refrigerants, hindered esters having excellent thermal resistance have been put to practical use. These hindered esters are derived from a carboxylic acid having a methyl branched chain or an ethyl branched chain in the α position or the β position and pentaerythritol. For example, Japanese Laid-Open Patent Publication No. 10-8084 discloses refrigeration oils containing an ester obtained from pentaerythritol and a mixed fatty acid of 2-ethylhexanoic acid and 3,5,5-trimethylhexanoic acid as the main component and discloses that such refrigeration oils have good stability at high temperatures. Moreover, Japanese Laid-Open Patent Publication No. 5-209181 discloses esters obtained from pentaerythritol, dipentaerythritol, and tripentaerythritol and discloses the viscosity and the compatibility with fluorocarbons of such esters. Furthermore, Japanese Laid-Open Patent Publication No. 6-330061 discloses refrigeration oils containing an ester obtained from pentaerythritol and a mixed fatty acid of a linear or branched fatty acid having 6 to 8 carbon atoms and 3,5,5-trimethylhexanoic acid as the main component. The publication discloses that such refrigeration oils have an excellent compatibility with refrigerants and have improved electric insulation properties. Furthermore, in order to improve lubricity and compatibility with refrigerants, Japanese Laid-Open Patent Publication No. 10-158215 discloses esters obtained from a fatty acid mixture and a polyhydric alcohol, wherein the fatty acid mixture includes 3,5,5-trimethylhexanoic acid and a saturated fatty acid other than 3,5,5-trimethylhexanoic acid, wherein the saturated fatty acid has an alkyl group as a side chain and has a total of 9 carbon atoms.
In a refrigerating cycle, a part of the refrigeration lubricant generally circulates through the cycle along with a refrigerant, and thus the refrigeration lubricant is exposed to a high temperature region and a low temperature region. As for high thermal resistance required in a compressor, which operates in the high temperature region, the above-described hindered esters, in particular, the mixed esters of the patent publications noted above satisfy the performance requirements. On the other hand, however, a part of the refrigeration lubricant ejected from the compressor may stagnate in the low temperature region. In particular, if the lubricant stagnates in the low temperature region for a long period of time, then it is crystallized and the amount of the circulating refrigerant in the refrigerating cycle is reduced, which may lead to problems such as poor refrigeration. Therefore, it is critical to the reliability of refrigerating apparatuses to develop a refrigeration lubricant higher stability in which no precipitation occurs over a long period of time even at low temperatures. However, the mixed esters obtained from a combination of a polyhydric alcohol and a specific carboxylic acid as described in the patent publications noted above have not been sufficiently examined for their ability of maintaining low-temperature stability over a long period of time. Thus, it is not possible to prevent crystallization from occurring in the low temperature region, and thus these esters lack long-term stability.